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A solar thermal collector is a solar collector designed to collect heat by absorbing sunlight. The term is applied to solar hot water panels, but may also be used to denote more complex installations such as solar parabolic, solar trough and solar towers. These complex collectors are generally used in solar power plants where solar heat is used to generate electricity by heating water to produce steam which drives a turbine connected to an electrical generator. A collector is a device for converting the energy in solar radiation into a more usable or storable form. The energy in sunlight is in the form of electromagnetic radiation from the infrared (long) to the ultraviolet (short) wavelengths. The solar energy striking the Earth's surface depends on weather conditions, as well as location and orientation of the surface, but overall, it averages about 1,000 watts per square meter under clear skies with the surface directly perpendicular to the sun's rays. Types of solar collectors for heat Solar collectors fall into two general categories: non-concentrating and concentrating. In the non-concentrating type, the collector area (i.e. the area that intercepts the solar radiation) is the same as the absorber area (i.e., the area absorbing the radiation). In these types the whole solar panel absorbs the light. Flat plate and evacuated tube solar collectors are used to collect heat for space heating or domestic hot water. Flat plate collectors Flat plate collectors, developed by Hottel and Whillier in the 1950s, are the most common type. They consist of (1) a dark flat-plate absorber of solar energy, (2) a transparent cover that allows solar energy to pass through but reduces heat losses, (3) a heat-transport fluid (air, antifreeze or water) flowing through tubes to remove heat from the absorber, and (4) a heat insulating backing. The absorber consists of a thin absorber sheet (of thermally stable polymers, aluminum, steel or copper, to which a black or selective coating is applied) backed by a grid or coil of fluid tubing placed in an insulated casing with a glass or polycarbonate cover. Fluid is circulated through the tubing to transfer heat from the absorber to an insulated water tank. This may be achieved directly or though a heat exchanger. Some fabricates have a completely flooded absorber consisting of two sheets of metal stamped to produce a circulation zone. Because the heat exchange area is greater they may be marginally more efficient than traditional absorbers. There are a number of absorber configurations: * harp - traditional design with bottom pipe risers and top collection pipe, used in low pressure thermosyphon and pumped systems * serpentine - one continuous S that maximises temperature but not total energy yield in variable flow systems, used in compact solar domestic hot water only systems (no space heating role) * completely flooded absorber consisting of two sheets of metal stamped to produce a circulation zone. Because the heat exchange area is greater they may be marginally more efficient than traditional absorbers. As an alternative to metal collectors, new polymer flat plate collectors are now being produced in Europe. These may be wholly polymer, or they may include metal plates in front of freeze-tolerant water channels made of silicone rubber. Polymers, being flexible and therefore freeze-tolerant, are able to contain plain water instead of antifreeze, so that they may be plumbed directly into existing water tanks instead of needing to use heat exchangers which lowers efficiency. By dispensing with a heat exchanger in these flat plate panel, temperatures need not be quite so high for the circulation system to be switched on, so such direct circulation panels, whether polymer or otherwise, can be more efficient, particularly at low light levels. However, polymer collectors suffer from overheating when insulated, as stagnation temperatures can exceed the melting point of the polymer.http://cat.inist.fr/?aModele=afficheN&cpsidt=17036823 http://www.springerlink.com/content/t632m867672l8w46/ For example, the melting point of polypropylene is 160°C, while the stagnation temperature of insulated thermal collectors can exceed 180°C if control strategies are not used. In areas where freezing is a possibility, metal collectors must be carefully plumbed so they completely drained down using gravity before freezing is expected so that they do not crack. Other collectors are part of a sealed heat exchange system, rather than having the potable water flow directly through the collectors. A mixture of water and propylene glycol (which is used in the food industry) can be used as a heat exchange fluid to protect against freeze damage, down to a temperature that depends on the proportion of propylene glycol in the mixture. The use of glycol lowers the water's heat carrying capacity only marginally, while the addition of an extra heat exchanger may lower system performance at low light levels. A pool or unglazed collector is simple form of flat-plate collector without a transparent cover. It is used for pool heating and can work quite well when the desired output temperature is near the ambient temperature (that is, when it is warm outside). As the ambient temperature gets cooler, these collectors become ineffective. Most flat plate collectors have a life expectancy of over 25 years. Evacuated tube collectors Evacuated tube collectors have multiple evacuated glass tubes which heat up solar absorbers and, ultimately, solar working fluid (water or an antifreeze mix—typically propylene glycol) in order to heat domestic hot water, or for hydronic space heating. The vacuum within the evacuated tubes reduce convection and conduction heat losses, allowing them to reach considerably higher temperatures than most flat-plate collectors. Two types of tube collectors are distinguished by their heat transfer method: the older type pumps a heat transfer fluid (water or antifreeze) through a U-shaped copper tube in each of the glass collector tubes. A newer type uses a sealed heat pipe that contains a liquid that vapourises as it is heated. The vapour rises to a heat-transfer bulb positioned outside the collector tube in a manifold through which water (in direct systems) or heat transfer fluid (HTF in indirect systems) is pumped. The vacuum that surrounds the outside of the tube reduces heat loss to the outside, therefore the greater efficiency of evacuated tube collectors. Therefore they can perform well in colder conditions. The advantage is largely lost in warmer climates, except in those cases where very hot water is desirable, for example commercial process water. The high temperatures that can occur may require special system design to avoid or mitigate overheating conditions though some have built in temperature limitation. The advantage over the flat-plate collectors is that the constant profile of the round evacuated tubes means that the collector is always perpendicular to the sun's rays and therefore the energy absorbed is approximately constant over the course of a day - provided the inner collecting tube is circular in section and not of the flat fin type. The question what to do with the "lost" sun shining through the gaps between evacuated tubes (gaps which can be as wide as the tubes' absorptive surface themselves) can be addressed either by adding specially curved metal reflectors under the evacuated tubes or by reverting to the use of flat plate collectors which are designed not to offer any gaps in the collector's heat interception profile. The gaps allow for snow to fall through the collector, minimizing the loss of production in some snowy conditions, though the lack of radiated heat from the tubes prevents effective shedding of accumulated snow. Evacuated tube collectors are made of a series of modular tubes, mounted in parallel, whose number can be added to or reduced as hot water delivery needs change. They have rows of parallel transparent glass tubes, each of which contains a collector tube (in place of the collector plate to which metal tubes are attached in a flat-plate collector). In some cases, the tubes are covered with a special light-modulating coating. In an evacuated tube collector, solar heat passing through an outer glass tube heats the absorber tube contained within it. The absorber can either consist of copper (glass-metal) or specially coated glass tubing (glass-glass). The glass-metal evacuated tubes are typically sealed at the manifold end, and the collector is actually sealed in the vacuum, thus the fact that the absorber and heat pipe are dissimilar materials creates no corrosion problems. Some systems use foam insulation in the manifold. Soda-lime glass is used in the higher quality evacuated tubes manufacture. Newer technology evacuated tube systems use a coated glass-and-metal absorber. The glass is a boron silicate material and the aluminum absorber plate and copper heat pipe are slid down inside the open top end of the tube. In lower quality systems moisture can enter the manifold around the sheet metal casing, and may eventually be absorbed by the glass fibre insulation and finds its way down into the tubes. This can lead to corrosion at the absorber/heat pipe interface area or freezing ruptures of the tube itself if the tube absorbs water. The high stagnation temperatures can cause antifreeze to break down, so care must be used if selecting this type of system in temperate climates. Tubes come in different levels of quality so the different kinds have to be examined as well. High quality units can efficiently absorb diffuse solar radiation present in cloudy conditions and are unaffected by wind. They also have the same performance in similar light conditions summer and winter. Comparisons of flat plate and evacuated tube collectors A long standing argument exists between protagonists of these two technologies. Some of this can be related to the physical structure of evacuated tube collectors which have a discontinuous absorbance area. An array of evacuated tubes on a roof has 1) open space between collector tubes and 2) (vacuum-filled) space occupied between the two concentric glass tubes of each collector tube. Consequently, a square meter of roof area covered with evacuated tubes (collector gross area) is larger than the area comprising the actual absorbers (absorber plate area). If evacuated tubes are compared with flat-plate collectors on the basis of square meterage of roof occupied, a different conclusion might be reached than if the areas of absorber were compared. In addition, the way that the ISO 9806 standard ISO 9806-2:1995. Test methods for solar collectors -- Part 2: Qualification test procedures. International Organization for Standardization, Geneva, Switzerland specifies the way in which the efficiency of solar thermal collectors should be measured is ambiguous, since these could be measured either in terms of gross area or in terms of absorber area. Flat-plate collectors suffer from the disadvantage of losing heat to the environment when the liquid in the collector reaches a temperature far above the ambient. Evacuated tube collectors suffer the disadvantage that their absorber plate area to gross area ratio is smaller (typically 60-80% of gross area) than for flat plates. In early designs the absorber area only occupied about 50% of the collector panel. However this has changed as the technology has advanced to maximize the absorption area. Based on absorber plate area, many evacuated tube systems are more efficient than equivalent flat plate systems. In general, evacuated tubes work well when the ambient temperature is low (e.g. during winter) or when the sky is overcast for long period. On the other hand, in areas with much sunshine and solar heat, a flat plate collector may be more efficient especially if its price compares favorably with that of an evacuated tube collector. Although several European companies manufacture evacuated tube collectors, the evacuated tube market is dominated by manufacturers in the East. Several Chinese companies have long favorable track records and have been manufacturing these devices for 15–30 years. There is no unambiguous evidence that the two collector technologies (flat-plate and evacuated tube) differ in long term reliability. However, the evacuated tube technology is younger and (especially for newer variants with sealed heat pipes) still need to prove equivalent lifetimes of equipment when compared to flat plates. The modularity of evacuated tubes yields clear advantages in terms of extendability and maintenance. For a given absorber area, evacuated tubes can therefore maintain their efficiency over a wide range of ambient temperatures and heating requirements. In extremely hot climates, flat-plate collectors will generally be a more cost-effective solution than evacuated tubes. When employed in arrays of 20 to 30 or more, the efficient but costly evacuated tube collectors have a net benefit in winter and also give real advantage in the summer months. They are well suited to cold ambient temperatures and work well in situations of consistently low sunshine, providing heat more consistently than flat plate collectors. On the other hand, heating of water by a low amount (i.e. low Tm-Ta) is much more efficiently performed by flat plate collectors. For instance this makes flat plate collectors superior devices for heating swimming pool water, and in tropical or subtropical environments where household water needs to be heated by less than 20°C. Air These collectors heat air directly, almost always for space heating. They are also used for pre-heating make-up air in commercial and industrial HVAC systems Types of solar collectors for electricity generation Parabolic troughs, dishes and towers described in this section are used almost exclusively in solar power generating stations or for research purposes. The conversion efficiency of a solar collector is expressed as eta0 or η0. Parabolic trough This type of collector is generally used in solar power plants. A trough-shaped parabolic reflector is used to concentrate sunlight on an insulated tube (Dewar tube) or heat pipe, placed at the focal point, containing coolant which transfers heat from the collectors to the boilers in the power station. Parabolic dish It is the most powerful type of collector which concentrates sunlight at a single, focal point, via one or more parabolic dishes—arranged in a similar fashion to a reflecting telescope focuses starlight, or a dish antenna focuses radio waves. This geometry may be used in solar furnaces and solar power plants. There are two key phenomena to understand in order to comprehend the design of a parabolic dish. One is that the shape of a parabola is defined such that incoming rays which are parallel to the dish's axis will be reflected toward the focus, no matter where on the dish they arrive. The second key is that the light rays from the sun arriving at the Earth's surface are almost completely parallel. So if dish can be aligned with its axis pointing at the sun, almost all of the incoming radiation will be reflected towards the focal point of the dish—most losses are due to imperfections in the parabolic shape and imperfect reflection. Losses due to atmosphere between the dish and its focal point are minimal, as the dish is generally designed specifically to be small enough that this factor is insignificant on a clear, sunny day. Compare this though with some other designs, and you will see that this could be an important factor, and if the local weather is hazy, or foggy, it may reduce the efficiency of a parabolic dish significantly. In some power plant designs, a stirling engine coupled to a dynamo, is placed at the focus of the dish, which absorbs the heat of the incident solar radiation, and converts it into electricity. Power tower A power tower is a large tower surrounded by small rotating (tracking) mirrors called heliostats. These mirrors align themselves and focus sunlight on the receiver at the top of tower, collected heat is transferred to a power station below. Solar pyramids Another design is a pyramid shaped structure, which works by drawing in air, heating it with solar energy and moving it through turbines to generate electricity. Solar pyramids have been built in places like Australia. Currently India is building such pyramids.Solar Pyramids Advantages * Very high temperatures reached. High temperatures are suitable for electricity generation using conventional methods like steam turbine or some direct high temperature chemical reaction. * Good efficiency. By concentrating sunlight current systems can get better efficiency than simple solar cells. * A larger area can be covered by using relatively inexpensive mirrors rather than using expensive solar cells. * Concentrated light can be redirected to a suitable location via optical fiber cable. For example illuminating buildings. * Heat storage for power production during cloudy and overnight conditions can be accomplished, often by underground tank storage of heated fluids. Molten salts have been used to good effect. Disadvantages * Concentrating systems require sun tracking to maintain Sunlight focus at the collector. * Inability to provide power in diffused light conditions. Solar Cells are able to provide some output even if the sky becomes a little bit cloudy, but power output from concentrating systems drop drastically in cloudy conditions as diffused light cannot be concentrated passively. Standards *ISO 9806: Test methods for solar collectors. * EN 12975: Thermal solar systems and components. Solar collectors. * EN 12976: Thermal solar systems and components. Factory made systems. *EN 12977: Thermal solar systems and components. Custom made systems. See also *Solar hot water *Solar thermal energy *Insulated glazing *Selective surface *Solar collector *Solar oven *Solar heating *Solar Flower Tower *Seasonal thermal store *Trombe wall *Zeolite References *The US ratings of solar thermal collectors, updated regularly—flat plate, evacuated, air and pool collectors rated *[[Sorption] Materials for Application in Solar Heat Energy Storage] *Hottel, H. C. and Whillier, A.: "Evaluation of Flat-Plate Solar Collector Performance," Trans. of the Conference on the Use of Solar Energy - The Scientific Basis, Vol. 2, Tucson, AZ, Oct. 31- Nov. 1, 1955, pp 74–104. External links * formula for calculating efficiency of solar collector * Parabolic Light Collector * Online Simulation of Non-Tracking Solar Thermal Collector Power Output nl:Zonneboiler ru:Солнечный коллектор sv:Solmaskin tr:Güneş enerjisi uk:Сонячний колектор Category:Solar powered devices Category:Solar thermal